Can you explain why it works better with 2+ players? Is it because the game is harder to win if you play solo, or is it because there are interactions between players that are an essential part of the game?

this is a cooperative game and it is clear that the interaction among players is giving spice to the game experience.

If you play solo, using two or three characters, you will miss big part of this experience because you will apply your own strategy and you will not have to deal with other players choice or unexpected decisions or mistakes.